1. Field of the Invention
The present invention relates to an optical pickup device, and more particularly, to a readout optical system of an optical pickup device.
2. Background Art
A recording capacity of one layer in an optical disc largely depends on a wavelength of a used semiconductor laser and a numerical aperture (NA) of a used objective lens. As the wavelength of the semiconductor laser is shorter, or as the NA is larger, a recording density can be larger, and the capacity of one layer can be more increased. The mainstream of optical disc drives which are currently distributed in the market is a DVD (Digital Versatile Disc) drive which uses a red light having a wavelength in the vicinity of 650 nm and an objective lens having an NA of 0.6. Meanwhile, as an optical disc drive having a recording density larger than that of the DVD, there is also appearing in the market an optical disc drive which uses a blue-violet semiconductor laser having a light wavelength in the vicinity of 405 nm as its light source and an objective lens having an NA of 0.85. As a method of further increasing the recording density currently achieved, it is conceivable to use a laser having a shorter wavelength, but difficulties are anticipated in the development of a semiconductor laser in the ultraviolet region having a wavelength shorter than that of this blue-violet laser. In addition, with regard to providing the objective lens with a larger NA, the limit value of the NA of the objective lens in the air is 1, and hence it is also becoming difficult to improve the recording density by adjusting the NA of the objective lens.
In these circumstances, as a method of increasing the capacity of one optical disc, double-layering is carried out. Jpn. J. Appl. Phys., Vol. 42 (2003), pp. 956 to 960 introduces a technology for a double-layer phase-change disc. In a case of irradiating a double-layer optical disc with a laser light, an adjacent layer is also irradiated therewith at the same time, which causes a problem of an interlayer crosstalk. In order to reduce this problem, an interlayer spacing is made larger. Because the laser light is condensed, a layer other than an intended layer (target layer) is deviated from a condensing plane of the laser light, and hence the crosstalk can be reduced.
On the other hand, when the interlayer spacing is made larger, a problem of a spherical aberration arises. A recording layer is embedded in a polycarbonate having a refractive index different from that of the air, and the spherical aberration thereof differs depending on the depth from a disc surface. The objective lens is designed so that the spherical aberration thereof is small with respect to a specific layer. Therefore, when the focal point of the laser light is moved to another layer, the spherical aberration occurs because the distance of the focal position from the surface is different. This aberration can be normally corrected by disposing an expander lens optical system formed of two lenses or a liquid crystal element in front of the objective lens. That is, this aberration can be corrected by changing the distance between the two lenses or a phase of the liquid crystal element. However, in consideration of the compensable range of the liquid crystal element or the realization of a moving mechanism for the lenses within a small-sized optical disc drive device, it is difficult to correct a large spherical aberration.
In a case where multi-layering is to be carried out in order to further increase the capacity, a total thickness of the multi-layer is restricted by the correction limit of the spherical aberration. As the number of layers is larger, the interlayer spacing becomes narrower. For this reason, the actual optical drive device for multi-layering still has an interlayer crosstalk unsolved.
In order to reduce the above-mentioned crosstalk, ISOM/ODS'08, Technical Digest Post-deadline Papers, TD05-1.55 (2008) utilizes that, when reflected lights from the multi-layer optical disc are condensed by a lens, condensing positions of the reflected lights from the intended layer and the adjacent layer are different on the optical axis. A grating is disposed so as to include the optical axis, and a reflecting mirror is disposed on a condensing plane of the reflected light from the target layer. The reflected light from the adjacent layer irradiates the grating, and thus is attenuated. On the other hand, the reflected light from the target layer is transmitted through the space between the grating and the reflecting mirror, and thus can return to a detection system without being attenuated. This makes it possible to reduce the interlayer crosstalk.
In addition, in Jpn. J. Appl. Phys., Vol. 45, No. 2B (2006) pp. 1174 to 1177, a tracking signal is obtained by using a single beam, and a stray light from the double-layer is prevented from affecting the tracking signal. The structure is adopted, in which the light in a central part of the grating disposed in a return path is detected outside of the optical axis, whereby the stray light is prevented from entering a quadripartite detector for detecting a tracking signal which is disposed in the vicinity of the center of the optical axis.